I haven's seen the default styles for any other theme but Classic available online but if you happend to find them, please post back here :) I think the templates over at MSDN are usually the Classic theme.
You can use Expression Blend to get it, you can download a trial here.
Select the CheckBox
, go to Object -> Edit Style -> Edit a Copy.
Assuming you are after the windows 7 (aero) style, here it is
<SolidColorBrush x:Key="CheckBoxFillNormal" Color="#F4F4F4"/>
<SolidColorBrush x:Key="CheckBoxStroke" Color="#8E8F8F"/>
<Style x:Key="EmptyCheckBoxFocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="1" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="CheckRadioFocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="14,0,0,0" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="CheckBoxStyle1" TargetType="{x:Type CheckBox}">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="Background" Value="{StaticResource CheckBoxFillNormal}"/>
<Setter Property="BorderBrush" Value="{StaticResource CheckBoxStroke}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="FocusVisualStyle" Value="{StaticResource EmptyCheckBoxFocusVisual}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CheckBox}">
<BulletDecorator Background="Transparent" SnapsToDevicePixels="true">
<BulletDecorator.Bullet>
<Microsoft_Windows_Themes:BulletChrome BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" IsChecked="{TemplateBinding IsChecked}" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderPressed="{TemplateBinding IsPressed}"/>
</BulletDecorator.Bullet>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</BulletDecorator>
<ControlTemplate.Triggers>
<Trigger Property="HasContent" Value="true">
<Setter Property="FocusVisualStyle" Value="{StaticResource CheckRadioFocusVisual}"/>
<Setter Property="Padding" Value="4,0,0,0"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>